Mercury (Hg) emissions have become a health and environmental concern because of their toxicity and ability to bioaccumulate. The U.S. Environmental Protection Agency (EPA) has issued regulations for the control of Hg emissions from waste-to-energy, cement production, and coal-fired power plants. Mercury in flue gas from industrial sources (e.g., power plants) can be captured by injection of sorbents such as activated carbon, which can then be removed by particulate collection devices. The amount of standard sorbents (e.g., activated carbon) needed to serve the market is large. Standard sorbents are not always effective and become more expensive as injection rates increase.
A nanocomposite is a multiphase solid material in which one of the phases has at least one dimension of less than about 1000 nm or in which less than an about 1000 nm repeat distance separates the phases that make up the material. Nanocomposites in which one or more of the phases is a bulk matrix and one or more other materials is a nanodimensional phase can have unique properties, with the mechanical, electrical, thermal, optical, chemical, or catalytic properties of the nanocomposite material differing significantly from that of the individual component materials.
The first examples of carbon nanocomposites were prepared by intercalating monomers into interlamellar spaces in clays, polymerizing the monomer, and carbonizing the polymer. The minimum thickness of the carbon layer was 1.1 nm (Kyotani-1988). In the next two decades, a variety of monomers were employed with several clays and other porous support materials. In some cases, the inorganic part of the composite was removed to study the graphite-like carbon structures. In 2004, Bakandritsas et al. produced carbon-clay nanocomposites using sucrose as the carbon source (Bakandritsas-2004). The thickness of each layer was about 1 nm and 0.4 nm for the clay and graphene layers, respectively. Later, this group described the use of these for adsorption of gases (CO2, CH4, N2) and organic solutes in aqueous solutions (Bakandritsas-2005). Because they can be easily shaped, have high surface-areas, and conduct electricity, carbon-clay nanocomposites from sucrose were used to produce electrodes and sensors (Darder-2005, Gomez-Aviles-2007, Fernandez-Saavedra-2008, Gomez-Aviles-2010). The porous carbon-clay nanocomposites from sucrose also have been utilized for catalyst supports (Nguyen-Thanh-2006a. Nguyen-Thanh-2006b, Ikeue-2008).
Several applications of composite materials for adsorption of metal ions such as Hg2+ have been described in the literature. These include the following materials: chitosan-coated ceramic (Boddu-2002), polypyrrole film on clay (Eisazadeh-2007), mercapto-functionalized polysiloxane film on diatoms (Wang-2007), polyaniline film on ash (Ghorbani-2011), and polyaniline composite with humic acid (Zhang). None of these is a carbon nanocomposite; rather, they are typically a polymer film deposited on a support and suffer limitations from stability and difficulty of recycling and processing.
Separation of elemental or oxidized mercury from a gas stream has been conducted with several types of nanocomposites made with non-carbon materials. A SiO2—TiO2 nanocomposite was used for Hg capture under UV radiation (Li-2008). This technology suffers from the difficulty of having to effectively irradiate combustion effluent containing fine particulate. A magnetite- and Ag-impregnated zeolite nanocomposite was described (Dong-2009). It is suspected that the Ag nanocomposite represents a significant environment risk in itself, as well as being a high-cost sorbent. Capture of Hg in flue gas with a CeO2—WO3/TiO2 nanocomposite was reported (Wan-2011). These non-carbon sorbents have higher cost and slower kinetics than desirable.